Recently, the problems of pathogenic microorganisms such as Cryptosporidium and O-157 in addition to the worldwide shortage of water become more serious, and there is demanded a water purification process which can simply produce water high in safety and quality. The microfiltration (MF) and ultrafiltration (UF) using filter elements comprising a filter medium of porous body have been noticed as water purification processes which can effectively remove suspended materials and harmful materials such as pathogenic microorganisms in liquid by a simple operation. As the filter elements used for microfiltration or ultrafiltration, there are generally used those which have partition walls comprising a porous body such as resin or ceramics to form cells as flow paths of liquid.
For example, a filter of hollow fiber membranes is a filter element comprising a large number of hollow fibers as filter media and has capillary partition walls comprising a porous resin, which form cells piercing through the central portion. In this structure, when a liquid to be treated (unfiltered liquid) is supplied to the outside of the hollow fibers under a given pressure, the liquid permeates through the partition walls comprising a porous resin and flows into the cells piercing through the central portion of the hollow fibers. In this case, suspended materials and harmful materials such as pathogenic microorganisms in liquid are removed through the partition walls, and the liquid flowing into the cells can be recovered as a treated liquid (filtrate). That is, in the filter of hollow fiber membrane, the cells formed by partition walls are utilized as flow paths through which filtrate is flowed.
The monolithic filter 2 shown in FIG. 1 is a filter element in which a ceramic porous body is used as a filter medium and a large number of cells 3 are formed so that the directions of flow paths for liquid are in parallel. This filter element has a honeycomb structure which has lattice-like partition walls comprising a ceramic porous body, and a large number of cells 3 defined by the partition walls. In this structure, when a liquid to be treated (unfiltered liquid) is supplied to the inside of cells 3 under a given pressure, the liquid permeates through the partition walls comprising a ceramic body and flows out of the cells. In this case, suspended materials and harmful materials such as pathogenic microorganisms are removed through the partition walls, and the liquid which flows out of the cells, namely, out of the monolith, can be recovered as a treated purified liquid (filtrate). That is, in the monolithic filter 2 shown in FIG. 1, cells 3 formed by the partition walls are utilized as flow paths through which unfiltered liquid is allowed to flow.
When the above filter element is used for microfiltration or ultrafiltration, in many cases, it is used in the form of a filter comprising the filter element is contained in a casing. All of the above-mentioned filter elements have a structure where the flow paths for unfiltered liquid and the flow paths for filtrate are divided by the partition walls, though there is a difference whether the cells are used as flow paths for filtrate or as flow paths for unfiltered liquid. Therefore, it becomes possible to separate and recover only the filtrate by containing the filter element in a casing, and the flow paths for unfiltered liquid and the flow paths for filtrate are liquid tightly isolated with a sealing material such as O-ring. Generally, in such a filter the filter element is contained in the casing so that the liquid is allowed to flow in vertical direction.
The above filter singly has a filtrating function, but in order to increase filtration area, namely, increase water permeation amount per unit time and unit filtration area (hereinafter referred to as merely “water permeation amount”) and improve filtering ability in a large-scale plant such as water purification plant or factory which is required to treat a large amount of unfiltered liquid, the filter is generally used as a filtration device having a structure where a large number of the filters are connected with each other by piping. For example, there is known a filtration device having a structure where a large number of filters are connected by header tubes (e.g., Patent Document 1).    Patent Document 1: JP-A-2003-334429
However, the above filtration device having a structure where a large number of filters are connected by header tubes is useful for increase of filtration area and improvement of treating ability, but has the problem that a sufficient washing effect cannot be obtained at the time of back washing.
When filtration is continuously carried out with the above-mentioned filter, suspended materials and the like which are filtered off are gradually deposited on the surface of the partition walls of the filter element, and hence the water permeability gradually lowers. Therefore, washing operation called “back washing” which comprises passing under pressure a clarified water or washing liquid chemical in opposite direction to filtration direction (namely, toward the side of unfiltered liquid flow paths from the side of filtrate flow paths) is carried out periodically or non-periodically. By this back washing, suspended materials and the like deposited on the surface of the partition walls of the filter element can be separated and removed, and the water permeability can be recovered to nearly the initial level.
However, in the case of the above filtration device, it is difficult to uniformly apply the back washing pressure to a large number of the filters which constitute the filtration device due to the device structure where a large number of filters are connected by header tubes, and there is the possibility of washing effect in respective filters being non-uniform. Similarly, in one filter, it is also difficult to uniformly apply the back washing pressure to a large number of cells formed in the filter element, and sometimes there are variations in the washing effect in respective cells.
Under the circumstances, since back washing effect is apt to be insufficient, the above filtration device are not preferred because of various problems that (i) the filtration area which the filter element inherently has cannot be effectively utilized and the water permeability decreases than design value and (ii) even if the back washing is once carried out, the water permeability decreases again in a short time and the back washing must be frequently carried out.